A little late to the party, but I think the ending was less lackluster than first presented.
Once Alice returns to the real world, she must return to the place she first appeared in the dream world in order to fully unlock her memories. She does so soon after she recovers from her injuries, and is able to prevent the bomb from going off. The plot should have ended here, however we are given several interesting pages afterwards.
As soon as Alice recovers her memories, there is a scene presented as two bust shots: first of Kiina, who has a sad smile on her face, and then of Alice, who shouts her name in tears, as if calling out to her friend. Then, the following pages are shots of Alice enjoying everyday life, and then finally announcing "that was fun!"
The interesting thing about the time Alice spends playing is that her eyes are obscured during nearly the entire time she's enjoying sightseeing. Around her, everyone -and I mean everyone- is either physically with someone else, or communicating with someone else. Alice is the only one who is truly "alone." Her being the ONLY one alone, in this case, is indicative that she is not really alone.
Alice's eyes are obscured up until the very end. You can clearly see her "normal" eyes as she takes pictures with the geisha in the third to last page of manga, however the style of her eyes drastically changes in the last shot of Alice as she looks at the many groups of people around her. Her expression, as well as the drawn style of her eyes are nearly identical to that of Kiina in her earlier bust shot.
There are also other small hints that this is what happened: on page 41, the last panel shows Alice walking past the old woman with kids from Kiina's memory, on the side of the arm engraved with "Kiina."
On page 40, 41, & 42, Alice is shown doing three activities: buying gifts, trying on wigs, and then taking pictures. In each of these situations, the left hand, the hand with Kiina written in it, is always shown first. On 40, Alice is shown peering into the shop with her left hand facing forward, and then later with her right hand clutching the gift. On 41, Alice can be seen ordering with her left hand, but then eating with her right. On 42, Alice notices the geisha with her left hand forward, and then takes a picture with her right. Such a thing would make sense if Alice and Kiina were together- Kiina wants to try something, but is to socially unadjusted to go through with it. Alice then takes over and executes her wishes.
The title page should be an even bigger indicator.
Alice and Kiina sharing a body is not too implausible, as two of characters - the pair of sisters, Ayon and Hayon - already did so in the dream world. After all, Alice and Kiina are "sisters" now too, right?